Gradient Materials

July 28, 2014

It's unknown who coined the expression, "Change is good," and that's a good thing, since he would be a hated man. No one likes change, and you can be certain that when you hear some corporate manager say, "Change is good," bad things are about to happen. If there must be change, people would like the change to be gradual. A decade time scale, preferably a lifetime, would be nice.

where n1 and n2 are the two refractive indices (in any order), gives you a reflectanceR of about 4% at normal incidence. Adding an intermediate layer with a refractive index of 1.225 gives 1% at each interface, so the reflectance is halved.

An example of a graded material. A multimode optical fiber with a graded-index core has lower loss than a step-index fiber. (Illustration by Stanisław Skowron, via Wikimedia Commons.)

Often it's advantageous to have a surface that's highly strained. In a process called shot peening, or ball peening, an alloy's surface is pelted by hard metal balls to induce a compressive surface strain. The strain profile is gradual through the alloy, since there are more impact forces at the surface than below the surface.

The purpose of such a layer is to enhance fracture toughness, since the compression tends to close any cracks at the surface before they can grow larger. Many years ago I was a member of a research team that used compressive layers on articles made from single crystals to increase their fracture toughness.[1-3]

As I've often mentioned, a material derives its properties from both its chemical composition and its microstructure. Most materials are polycrystalline; that is, they are composed of a myriad of small single crystals packed together, and the size and orientation of these grains determine many of the material properties, including its strength and fracture toughness. Smaller grains offer hardness, and larger grains offer ductility.

Some natural materials derive their excellent mechanical properties from a hierarchical structure, mixing smaller and larger structures to give both strength and ductility. I wrote about the hierarchical structure of spider silk in a previous article (Spider Silk, March 12, 2012). What the Chinese-North Carolina research team did was to create materials with a gradient in grain size from large grains in the interior to small grains at the surface to derive benefit from their different properties.[6]

The research team tried this gradient approach on many metals, including copper, iron, nickel and stainless steel, and the mechanical properties were improved for them all.[6] Experiments were also done for interstitial-freesteel. Although this material can be made with 450 MPatensile strength, it has low ductility, having a strain limit of about 5%. The gradient process produced material with 500 MPa tensile strength, and a 20% rupture strain.[6]

These improved mechanical properties arise from conversion of an applied uniaxial stress to bi-axial stress generated by the gradient. This results in the the accumulation and interaction of dislocations that leads to additional work hardening that supplements that of non-gradient materials. The gradient technique is a novel path to superior mechanical properties for an otherwise optimized material.[4-5]